Monitored Item

The parent object that manages a collection of Monitored Items. A Subscription defines the publishing interval and lifetime for a group of data change notifications. The Client creates a Subscription within a Session, and the Server uses it to bundle and pace the delivery of notifications from all associated Monitored Items. Key parameters include:

• Publishing Interval: The cyclic rate at which the Server sends Notification Messages.
• Keep-Alive Count: The number of publishing intervals that may elapse without data before a keep-alive message is sent.
• Lifetime Count: The number of publishing intervals without a response before the Subscription is terminated.

A data filter applied directly to a Monitored Item to suppress noise and reduce network traffic. When configured, the Server calculates the absolute change in a numeric value since the last notification. If the change is less than the defined deadband threshold, no notification is generated. OPC UA defines two types:

• Absolute Deadband: Triggers only when the absolute difference exceeds a fixed value.
• Percent Deadband: Triggers based on a percentage of the EURange property of the Variable Node. This is critical for analog sensor signals that naturally fluctuate within an acceptable noise floor.

A per-Monitored Item setting that defines the fastest rate at which the Server evaluates the underlying Node attribute for a change. This is distinct from the Subscription's publishing interval. The Server samples at this rate, applies any filters, and queues the result for the next publishing cycle. A sampling interval of -1 instructs the Server to use the publishing interval. A value of 0 tells the Server to sample as fast as possible, using the fastest practical rate for the underlying data source, which is essential for high-speed discrete manufacturing signals.

The actual data packet sent from the Server to the Client during a publishing cycle. A single Notification Message contains a sequence number, a timestamp, and a list of DataChangeNotifications from all Monitored Items in the Subscription that have triggered since the last message. Each DataChangeNotification carries the new value, the status code (quality), and the source timestamp of the sampled attribute. The Client acknowledges receipt to keep the Subscription alive and maintain the reliable delivery sequence.

The OPC UA service set that provides the foundational read, write, and monitor operations for real-time process data. The Monitored Item is a core component of the Data Access monitoring model. Beyond simple value monitoring, Data Access defines the StatusCode structure that accompanies every value change, indicating data quality (Good, Uncertain, Bad) and sub-status details like sensor failure or calibration state. This service set is the most widely implemented part of OPC UA in discrete and process manufacturing.

A stateful eventing model that extends beyond the simple value monitoring of a Monitored Item. While a Monitored Item reports a value change, an Alarm reports a state transition (e.g., inactive to active). Alarms require acknowledgment and confirmation workflows. A Condition is a broader concept that includes any system state with defined sub-states. Monitored Items can be configured to watch for EventNotifier attributes on Nodes, allowing Clients to subscribe to alarm state changes using the same Subscription infrastructure.